Unrestrained multi-planar or multi-leaded surface mount electronic components (e.g. power transistor) and ancillary devices (e.g. matching capacitors) tend to float in the emulsified solder during reflow soldering. As a results, the final position of these unrestrained devices is quite unpredictable. It is typical, therefore, that one or more of the component leads will fail to make proper contact with the circuit board, thereby rendering the circuit inoperative. In a similar fashion, imprecise ancillary device location adversely impacts upon circuit operation as well.
In response, the industry enlists the aid of complex spring loaded carriages or fixtures, designed to provide a clamping force upon the individual components throughout the reflow solder process. While this approach tends to resolve the problem of electronic component positioning, it nevertheless suffers from additional shortcomings.
The spring loaded carriages/fixtures are metal devices, considerably larger than the circuit boards that they house. Because of their metallic structure, the carriages/fixtures act like large heat sinks, thereby necessitating longer bake cycle times at higher temperatures in order to assure proper soldering. Unfortunately, the extended exposure represents a substantial threat to both temperature sensitive components and to solder joint integrity.
The spring loaded carriages are costly to manufacture and maintain. The primary manufacturing expense is tooling. Currently, each circuit board type requires one, some times two carriages to complete a reflow process. Each time a circuit board is redesigned, however, new carriages must be designed and manufactured in order to accommodate component repositioning. The added expense, however, often outweighs the original benefit anticipated by the circuit board redesign, thereby rendering such progresses impracticable. Additionally, severe maintenance costs arise from the rapid deterioration of these metal fixtures due to repeated use, temperature cycling, and the inevitable contamination and corrosion from oxidation, charring, and solder residue accumulations.
When ancillary devices are utilized, a stamped metal frame surrounding the electronic component is employed as a positioning guide. Its purpose is to maintain the relative position of the ancillary device to an electronic component lead, throughout the reflow solder process. Because these frames are metallic, however, they tend to draw solder away from the leads, thereby potentially compromising each lead connection. Additionally, these metal guides add sufficient thermal mass to compound the problem of longer bake cycle times at higher temperatures.
These and other shortcomings are endemic to metallic workpieces. While they tend to combat some of the problems associated with unrestrained reflow soldering, they are nevertheless plagued by troubles of a different nature. As previously discussed metallic workpieces represent a major expenditure in tooling and maintenance costs which severely burden PCB redesign efforts. In addition, the use of metallic workpieces during reflow soldering greatly increases oven cycle times. Moreover, the large spring loaded carriages/fixtures are not integratable into the ultimate PCB design, thus they must be removed prior to testing and operation.
It would therefore be extremely advantageous to provide a non-metallic device capable of providing the clamping function of a spring loaded carriage, the positioning function of the metallic frame guide, and yet adaptable for use with individual electronic components, thereby eliminating the need to redesign and manufacture new tooling after every new PCB layout.